It is known that isocyanates can be produced by reacting amines with phosgene. The reaction takes place via a carbamic acid chloride stage, the carbamic acid chlorides decomposing at elevated temperature into the corresponding isocyanates and hydrogen chloride. If the boiling point of the isocyanate to be produced is significantly above the decomposition temperature of the carbamic acid chloride, the hydrogen chloride formed during the decomposition can be removed from the reaction zone without difficulty, particularly where an inert organic solvent is used. If, however, the decomposition temperature of the carbamic acid chloride is close to, or above, the boiling point of the isocyanate, the isocyanate will enter the gas phase where it recombines with the hydrogen chloride to form carbamic acid chloride. Accordingly, decomposition is incomplete and the isocyanate obtained is contaminated with carbamic acid chloride.
The situation as outlined above applies to aliphatic monoisocyanates whose aliphatic radicals contain from 1 to 3 carbon atoms, the difficulties being greatest in the production of methylisocyanate.
Numerous processes which are intended to eliminate the difficulties referred to above are known and described in the art. Numerous publications describe the decomposition of carbamic acid chlorides using hydrogen chloride acceptors.
Thus, it is known that isocyanates can be produced from carbamic acid chlorides in the presence of organic bases (for example, tertiary amines) or carboxylic acid dialkylamides (German Offenlegungsschrift No. 1,593,554) or tetraalkyl ureas (U.S. Pat. No. 3,644,461) in organic solvents. In addition, German Auslegeschrift No. 2,156,761 describes the use of water, and British Pat. No. 1,208,862 describes the use of aqueous solutions or suspensions of inorganic bases, for absorbing the hydrogen chloride. Olefins have also been mentioned as hydrogen chloride acceptors (German Offenlegungschrift No. 2,210,285).
All the processes referred to above have the serious disadvantage that secondary products (corrosive organic or inorganic salts or alkyl chlorides) are formed, which either must be worked up at considerable expense or represent a source of atmospheric pollution. In addition, where organic bases are used, dimeric and trimeric isocyanates can be formed by secondary reactions. In the presence of water, a considerable proportion of the carbamic acid chloride is hydrolyzed to form the corresponding amine hydrochloride.
It is also known that the corresponding N-alkyl carbamic acid esters can be initially produced from the N-alkyl carbamic acid chlorides by reaction with aliphatic or aromatic hydroxyl compounds with the elimination and removal of hydrogen chloride. The isocyanates are subsequently obtained by thermally decomposing these carbamic acid esters (Houben-Weyl, Methoden der Organischen Chemie, Vol. 8, page 126, 1952).
For example, the corresponding monoisocyanates can be liberated by thermal decomposition from N-alkyl carbamic acid-2-hydroxy ethyl esters (U.S. Pat. No. 3,076,007) or from N-alkyl carbamic acid-.beta.-naphthyl esters (German Offenlegungsschrift No. 2,512,514).
The decomposition products of these processes partially recombine to form the carbamic acid esters upon completion of thermal decomposition. Additionally, non-volatile secondary products accumulate in the high-boiling hydroxyl compounds.
It is also known that low-boiling aliphatic monoisocyanates can be produced directly by the thermal decomposition of carbamic acid chlorides in organic solvents using special processing techniques.
According to German Auslegeschrift No. 1,193,034 and U.S. Pat. No. 3,388,145, thermal decomposition of the carbamic acid chloride is carried out in a reactor provided with a reflux condenser and separation column. Hydrogen chloride escapes through the reflux condenser, while isocyanate, carbamic acid chloride and solvent are retained. The isocyanate which is formed enters the column and can be removed at the head of the column. Most of the isocyanate is returned through a reflux divider so that the carbamic acid chloride ascending into the column passes back into the reactor.
Although this process is eminently suitable for the batch production of alkyl isocyanates, particularly on a laboratory scale, it is nevertheless attended by serious disadvantages which seriously complicate continuous working on a large scale. These disadvantages include the following:
1. Only a small proportion of the isocyanate present in the reactor and in the column can be removed at the head of the column. A large proportion of this isocyanate is removed from the reactor as a sump solution together with the residual non-decomposed carbamic acid chloride. If more product is removed at the head of the column, the product thus removed is not pure monoisocyanate. In that case, it is only possible to obtain mixtures containing considerable proportions of carbamic acid chloride at the head of the column. This disadvantage cannot even be overcome by using columns having a greater separation effect.
2. To obtain isocyanate free from carbamic acid chloride at the head of the column, the column has to be operated with high reflux ratios which involve a high consumption of energy.
3. For decomposing the residual, non-decomposed carbamic acid chloride, the isocyanate-containing carbamic acid chloride solution removed from the sump of the reactor must be returned to the reactor, optionally after part of the solvent has been separated. This inevitably results in a large isocyanate circuit which leads to very low volume-time yields.
4. Another effect of this large isocyanate circuit is that high concentrations of isocyanate are attained in the reactor. In that case, the readily volatile isocyanates are preferentially evaporated when the solutions are heated, so that the product vapors ascending to the reflux condenser have correspondingly high isocyanate contents. The ultimate result of this is that the effectiveness of the thermal removal of hydrogen chloride in the reflux condenser and, hence, the volume-time yield are drastically reduced.
The result of these disadvantages is that, where the above-mentioned process is carried out continuously, the reaction products and the solvent required for decomposing the carbamic acid chlorides have to be repeatedly recycled. The repeated evaporation and condensation of the mixtures, and the need for high reflux ratios in the distillation column of the reactor result in a high energy consumption and in serious losses of yield attributable to the formation of relatively high molecular weight derivatives and secondary products from the isocyanate and carbamic acid chloride (cf. for example H. Ulrich et al, J. Org. Chem. 29, 2401 (1964)).
German Offenlegungsschriften Nos. 2,411,441; 2,411,442; 2,422,211 and 2,503,270 describe process modifications which are also based on the principle of the above-mentioned process.
Thus, German Offenlegungsschrift No. 2,411,441 (corresponding to U.S. Pat. No. 3,969,389) describes a process in which the carbamic acid chloride is partly decomposed into isocyanate and hydrogen chloride by heating the carbamic acid chloride solution under reflux in a reactor equipped with a reflux condenser. The isocyanate thus formed is then isolated in a separate apparatus. The disadvantages referred to above apply to this process as well. In addition considerable outlay on apparatus is necessary.
German Offenlegungsschrift No. 2,411,442 (corresponding to U.S. Pat. No. 3,991,094) describes a process in which, during the thermal decomposition of the carbamic acid chloride under reflux, an inert gas stream is passed through the reaction mixture to remove the hydrogen chloride from the reaction zone. The decomposition of the carbamic acid chloride is accordingly promoted. In actual fact, however, this process does not produce any demonstrable increase in the decomposition of the carbamic acid chloride because the inert gas stream does not have a selective rectifying effect. Thus, not only are increased quantities of hydrogen chloride removed from the reaction zone with the inert gas, but also corresponding increased quantities of low-boiling isocyanate. In addition, the process involves a considerable outlay on apparatus.
The above-mentioned disadvantages also apply to the process described in German Offenlegungsschrift No. 2,503,270. In the first stage of this multistage process, a carbamic acid chloride solution is treated by thermal decomposition under reflux. The solution thus formed is heated again under reflux while an inert gas stream is passed through, the residual carbamic acid chloride being converted into isocyanate.
German Offenlegungsschrift No. 2,422,211 (corresponding to U.S. Pat. No. 3,969,388) describes a process in which the removal of hydrogen chloride from the carbamic acid chloride is carried out by heating the solutions under reflux in 2 to 6 successive reaction zones, followed by isolation of the isocyanate. However, no detectable quantities of hydrogen chloride are eliminated in this process either in the second reaction zone or in the following reaction zones. This could only be achieved by initially isolating isocyanate from the solutions removed from the reaction zones and subsequently introducing the solutions into the following reaction zone.
Finally, U.S. Pat. No. 4,082,787 describes a process whereby a solution of methyl carbamyl chloride in a nonpolar solvent is thermally dehydrochlorinated. The gases formed are condensed at a temperature above the boiling point of methyl isocyanate. Hydrogen chloride gas is removed by condensing the remaining condensable gases at a temperature below the boiling point of methyl isocyanate. The methyl isocyanate is separated from the condensate of the first condensation step.
The object of the instant invention is to provide a process for the continuous processing of commercial solutions containing carbamic acid chloride and recovering the corresponding pure monoisocyanate in high yields, in which undesirably large product circuits, high product losses attributable to the formation of derivative and secondary products and an undesirably high consumption of energy are avoided.
According to the present invention, this object is achieved by the process described in detail in the following.